Sounds like a exhause restriction to me, but no one knows which bike you are asking about so we have to be generic in our investigation.
Let me mansplain how the exhause works so you can fix it yourself. On most engines, the exhaust ports stay closed because of a vacuum which has developed in the headers. The cause of this vacuum is several fold: First, a rarefaction was caused during the intake stroke when hot exhaust gases were sucked into the cylinders. Second, the heat sinking effect of the long mufflers (whose job it is to cool the exhaust in order to most efficiently extract spent implosion products) has caused the exhaust to cool during the intake/compression cycle. Third, the acoustical resonance of the pipes reinforces the vacuum pulses at certain frequencies.
In order to transfer the noise caused by a loose fitting on the carbochamber gasket flange, you need to have a propersized muffler bearing spanner or a 10.5mm 8 sided socket and a large screwdriver. After locating the fanotinizer sensor you will need to disconnect the reversion module and make sure it is grounded before opening the access panel.
Find the localizer flange and hold it while rotating the left sprocket couper located to the right side of the plenum adjuster. usually it looks like a Stanley gasket or earlier models have a longer shaft on the kirtsheimer flashing bracket that you need to remove with a 19/16 open sided socket or a short handle wrench.
After you should measure the size of the down pipe, if you look along the exhause you will see the carbon stained point where the leak is located and measure the approximate diameter of the pipe at that location. Using a pair of glassofler cutters open the top and bottom of a large beer can then slice it down the sides. Wrap it around the break in the pipe and use a pair of appropriately sized jubilee clamps to secure the can wrapped around the pipe.
Since the advent of permanently lubricated biturbo muffler bearings 50cc bikes have been boosted to 65 or even 70cc. This rating is based on the old Caster oil Containment measuring system where motorbike mufflers were rated by the capacity of the amount of oil they needed to contain to keep them lubricated. Modern technology has replaced the oil containing mufflers with specially designed 'expansion chambers', these get their names from being expanded but without having to contain extra muffler oil. These were developed by the invention of synthetic oil which is an odorless and colourless gas, further development has not given us totally oil free mufflers which has boosted the fuel milage to over 1000km per litre in Europe.
Muffler oil containment factor is always expressed as a ratio - 25:1, 33:1, 50:1, 75:1, 100:1 etc. This ratio is derived from taking the quantity of muffler oil rating based on the Caster Oil measuring system in cc's and multiply it times the cube root of the peak rpm rating of the expansion chamber and compile the result into the peak temperature of the combustion chamber.
For some machines the manufacturer has added a oil injection system, which uses a high pressure pump to inject the needed quantity of oil into the muffler bearings. After the oil has been through the bearing section there is a scavenge tube which will recover the oil and route it back to the storage tank. The convenience of this system provides the operator with not having to worry about the muffler oil containment factor, when the oil reservoir is depleted of oil a special monitoring system will lock up the piston from moving and shut the bike off. The operator then just needs to replenish the oil in the reservoir and unlock the piston. Because this only happens after many many miles of driving, most operators will sell their bikes rather then doing the proper maintenance required to unlock their piston.